This invention relates to a camera provided with a solid-state image pickup elements as image receiving means, and especially relates to a camera using as the image receiving means the solid-state image pickup elements which receives still images through an objective lens and converts them into digital signals.
Generally, in cameras provided with a solid-state image pickup elements, images projected through the objective lens on the active area of the image pickup elements such as a CCD (Charge Coupled Device) are photoelectrically converted into image signals and are further converted into digital signals.
Recently, in response to demand for high resolution capability, the pixel density of the CCD imager has increased while decreasing the overall size of the CCD size. As a result, the pixel pitch of the CCD imager is decreased considerably.
In addition, in order to optimize the light amount of the images incident on the CCD imager, an aperture regulator as aperture regulating means has been employed as same as those employed in a conventional camera for silver-halide light sensitive material.
It has been admitted that the smaller the pixel pitch of the CCD imager used as the solid-state image pickup elements for the image receiving means of the camera, the higher the resolution of the reproduced images is achieved as the number of pixels of CCD imager is increased.
It was revealed, however, that in the use of a small aperture to limit the light amount on the CCD imager, resolution and contrast of the reproduced images may be deteriorated due to the MTF (Modulation Transfer Function) deterioration caused by the diffraction in the objective lens system. This fact has impeded the improvement of the image quality of cameras as a whole.
In addition, in order to suppress the moire effect caused by the sampling pitch of the CCD imager, an optical low-pass filter is mounted in front of the CCD active area. It was also found that the combination of the influence of the diffraction by the small aperture and the elimination of high frequency components by the optical low-pass filter deteriorates the reproduced image quality under certain conditions.
The objective of the present invention is to provide a camera using solid-state image pickup elements as the image receiving means which dose not cause deterioration of the image quality, even if an aperture regulator is employed in order to regulate the light amount.
As a result of intensive study of the aforementioned problems, it is recognized that pertaining to the image quality, there is a specific relationship between pixel pitch of the solid-state image pickup elements and the F number of the aperture regulator, and also between the optical low-pass filter and the F number of the aperture regulator as well. On the basis of this study, the present invention provides a camera having the following structures which prevent the deterioration of image quality, even if an aperture regulator is employed in order to limit the light amount on the active area of the CCD imager.
1. A camera is characterized in that there are provided
an objective lens system which projects the image of a subject onto the focal plane,
an aperture regulating means which regulates the area of the aperture in order to limit the incoming light amount on the focal plane,
solid-state image pickup elements which are mounted on said focal plane and yields image signals by photo-electric conversion,
an optical low-pass filter which eliminates high frequency components of the incoming images incident on said solid-state image pickup elements, and
a controlling means which controls the filtering characteristics of said optical low-pass filter, wherein the pixel pitch of said solid-state image pickup elements is 7.0 xcexcm or less and said controlling means controls said optical low-pass filter in response to the F number of said aperture regulating means,
Herein, a control of said optical low-pass filter characteristics is defined as a changing of cutoff frequency range and/or filtering effect for high frequency components.
In the camera mentioned above, in the case that the pixel pitch of the solid-state image pickup elements is 7.0 xcexcm or less and an aperture regulator is employed in order to regulate the light amount, since the control means controls said optical low-pass filter in accordance with the F number of the aperture, the decline in resolution and contrast due to the diffraction effect may not be superimposed with the elimination of the high frequency components by the optical low-pass filter. This fact make it possible to minimize the deterioration of the image quality and to realize a camera using as the image receiving means the solid-state image pickup elements which do not degrade the image quality.
2. In the configuration described in item 1, said control means controls the filtering characteristics by inserting said optical low-pass filter into the optical path and/or removing it.
In the camera mentioned above, as an effect of the correlative control with the F number of the aperture by inserting and/or removing said optical low-pass filter, the decline of resolution and contrast due to the diffraction effect in the objective lens system could not be duplicated with the lack of the high frequency components suppressed by the optical low-pass filter, even if pixel pitch of the solid-state image pickup elements is less than 7.0 xcexcm and a mechanical aperture regulator is employed. This fact would make it possible to minimize the deterioration of the image quality and to realize a high resolution camera with a solid-state image pickup elements.
3. In the configuration described in item 2, said control means removes the optical low-pass filter from the optical path, in case the F number of said aperture regulator mean is more than 5.6.
In the camera mentioned above, as an effect of the correlative control with the F number of the aperture, wherein insertion and removal control of said optical low-pass filter is active, and said optical low-pass filter should be removed from the optical path when the F number of the aperture exceeds 5.6, the decline of resolution and contrast due to the diffraction effect in the objective lens system could not be duplicated with the lack of the high frequency components suppressed by the optical low-pass filter, even if pixel pitch of the solid-state image pickup elements is less than 7.0 xcexcm and a mechanical aperture regulator is employed. This fact would make it possible to minimize the deterioration of the image quality and to realize a high resolution camera with a solid-state image pickup elements.
4. In the configuration described in item 1, plurality of optical elements, each of which has a different effect by high frequency filtering, are provided for selective control of said low-pass filter. Said control means selects the optimum element in response to the F number of the aperture.
In the camera mentioned above, as an effect of the selective control of said low-pass filter in response to the F number of the aperture, the decline of resolution and contrast due to the diffraction effect in the objective lens system could not be duplicated with the lack of the high frequency components suppressed by the optical low-pass filter, even if pixel pitch of the solid-state image pickup elements is less than 7.0 xcexcm and a mechanical aperture regulator is employed. This fact would make it possible to minimize the deterioration of the image quality and to realize a high resolution camera with a solid-state image pickup elements.
5. In the configuration described in item 4, said control means selects one of optical elements or a combination of a plurality of them.
In the camera mentioned above, as an effect of the selective control of said low-pass filter in response to the F number of the aperture, wherein either one of the optical elements or a combination of a plurality of them would be selected, the decline of resolution and contrast due to the diffraction effect in the objective lens system could not be duplicated with the lack of the high frequency components suppressed by the optical low-pass filter, even if pixel pitch of the solid-state image pickup elements is less than 7.0 xcexcm and a mechanical aperture regulator is employed. This fact would make it possible to minimize the deterioration of the image quality and to realize a high resolution camera with a solid-state image pickup elements.
6. The present invention relates to a camera comprised of:
a) an objective lens system which projects the image of a subject onto the focal plane,
b) an aperture regulating means which controls the degree of aperture opening in order to limit the incoming light amount on the focal plane,
c) a solid-state image pickup elements which is mounted on said focal plane and yields image signals by the photo electric conversion,
d) an optical low-pass filter which eliminates high frequency components of the incoming images focused on said solid-state image pickup elements, and
e) a control means which inserts said optical low-pass filter into the optical path and/or removes it from the optical path. In addition, the pixel pitch of the solid-state image pickup elements should be less than 7.0 xcexcm, and the filtering characteristics of said optical low-pass filter should be controlled in response to the F number of said aperture regulator.
In the camera mentioned above, as an effect of the insertion and/or removal control of said low-pass filter in the optical path, the decline of resolution and contrast due to the diffraction effect in the objective lens system could not be duplicated with the lack of the high frequency components suppressed by the optical low-pass filter, even if pixel pitch of the solid-state image pickup elements is less than 7.0 xcexcm and a mechanical aperture regulator is employed. This fact would make it possible to minimize the deterioration of the image quality and to realize a high resolution camera with a solid-state image pickup elements.
7. In the configuration described in item 6, said control means removes the optical low-pass filter from the optical path, in case the F number of said aperture regulating means is more than 5.6.
In the camera mentioned above, as an effect of the correlative control with the F number of the aperture, wherein insertion and remove control of said optical low-pass filter is active and said optical low-pass filter should be removed from the optical path when the F number of the aperture exceeds 5.6, the decline of resolution and contrast due to the diffraction effect in the objective lens system could not be duplicated with the lack of the high frequency components suppressed by the optical low-pass filter, even if pixel pitch of the solid-state image pickup elements is less than 7.0 xcexcm and a mechanical aperture regulator is employed. This fact would make it possible to minimize the deterioration of the image quality and to realize a high resolution camera with a solid-state image pickup elements.
8. In the configuration described in item 6, a plurality of optical elements, each of which has a different high frequency filtering effect, are provided for selective control of said low-pass filter. Said control means would insert said optical low-pass filter into the optical path in response to the F number of the aperture.
In the camera mentioned above, as an effect of the selective control of said low-pass filter in response to the F number of the aperture, the decline of resolution and contrast due to the diffraction effect in the objective lens system could not be duplicated with the lack of the high frequency components suppressed by the optical low-pass filter, even if pixel pitch of the solid-state image pickup elements is less than 7.0 xcexcm and a mechanical aperture regulator is employed. This fact would make it possible to minimize the deterioration of the image quality and to realize a high resolution camera with a solid-state image pickup elements.
9. In the configuration described in item 8, said control means would insert one of said optical low-pass filters or a combination of a plurality of them.
In the camera mentioned above, as an effect of the selective control of said low-pass filter in response to the F number of the aperture, wherein either one of the optical elements or a combination of a plurality of them would be selected, the decline of resolution and contrast due to the diffraction effect in the objective lens system could not be duplicated with the lack of the high frequency components suppressed by the optical low-pass filter, even if pixel pitch of the solid-state image pickup elements is less than 7.0 xcexcm and a mechanical aperture regulator is employed. This fact would make it possible to minimize the deterioration of the image quality and to realize a high resolution camera with a solid-state image pickup elements.